1. Field of the Invention
The present invention relates to a semiconductor device that includes an Insulated Gate Bipolar Transistor (referred to as IGBT) having a trench structure.
2. Description of Related Art
An example of the IGBT having a trench structure is disclosed in JP-A-2001-308327. In this type of IGBT, a channel density can be made larger than that in a so-called IGBT having a planar structure. In other words, a saturated voltage between an emitter and a collector (ON-voltage) can be made lower in the IGBT having a trench structure.
It has been generally known that an avalanche phenomenon tends to be caused in the IGBT at a turning-off under a high current density. If the avalanche phenomenon occurs, the IGBT may be broken by a breakdown current. Especially in the IGBT having a trench structure, there is a tendency that a breakdown current concentrates at a bottom surface of a gate trench due to concentration of electric field at the bottom surface of the gate trench, and a gate insulating film is broken. This is confirmed also by inventors of the present invention.